The primary research objective of this proposal is the development of an experimental system for the temporal and tissue-specific control of gene expression in the mouse urothelium. The model system will be validated by demonstration of controlled expression of the erbB2 oncogene. The tetracycline control system will be used in a bi-transgenic strategy. One of the two transgenic mouse strains TRE-erbB2 has been used to show erbB2 dependent hyperplasia of epithelial tissues. This mouse strain is available for these studies. The second transgenic mouse strain will have the reverse tetracycline transactivator (rtTA) gene under control of the uroplakin II (UPII) promoter for urothelium specific expression. In crosses between the TRE-erbB2 and UPII-rtTA mice the rtTA protein will be used to up regulate transcription of the TRE-erbB2 transgene in the presence of the tetracycline related antibiotic, doxycycline. In this way the expression of the erbB2 oncogene will be tissue specific and regulated in time by administration of doxycycline in drinking water. Each of the component parts of this system have been validated independently and will be assembled together here as a valuable tool for the investigation of urothelial growth, differentiation, and tumorigenesis. The UPII-rtTA transgenic mouse may also be crossed to control other genes placed under control of the TRE. The specific aims of this application are: Specific aim 1:Characterize UPII-rtTA transgenic mice and establish homozygous lines. Specific Aim 2: Determine the impact on mouse urothelium of doxycycline exposure in UPII-rtTA and TRE-erbB2 transgenic mice. Specific aim 3: Determine the impact of activated erbB2 expression on cellular proliferation and tumorigenesis in the mouse urothelium.